Niobium powder, sintered body thereof and capacitor using the same

ABSTRACT

A niobium powder having a nitrogen content of about 500—about 7,000 ppm by weight, and having a mean particle diameter of at least about 0.2 μm and less than about 3 μm. Preferably the niobium powder has a reduced content of impurities. A sintered body of the niobium powder. This sintered body generally has a specific leakage current index of not more than about 400 [pA/(μF·V)]. The capacitor having (i) an electrode composed of the sintered body, (ii) a counter electrode and (iii) a dielectric intervening between the two electrodes exhibits good leakage current characteristics.

This is a Continuation-in-part of PCT/JP00/04753 filed on Jul. 23, 2001,and U.S. application Ser. No. 09/636,638 filed Aug. 11, 2000, whichclaims benefit of Provisional Application No. 60/148,265 filed Aug. 11,1999, with the disclosures of the above noted prior applications beingall incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a niobium powder used for a capacitor havingexcellent leakage current characteristics; a sintered body produced fromthe niobium powder; and a capacitor having the sintered body.

2. Description of the Related Art

Capacitors to be incorporated in electronic apparatuses such as portablephones and personal computers are demanded to have a small size and ahigh capacitance. Among such capacitors, a tantalum capacitor has beenwidely used, in view of high capacitance relative to its size, andexcellent performance. Generally, in the tantalum capacitor, a sinteredbody of tantalum powder is used as a positive electrode, and therefore,in order to increase the capacitance of the capacitor, the weight of thesintered body must be increased.

When the weight of the sintered body is increased, the capacitornecessarily becomes larger in size and fails to satisfy the demand for asmall-sized capacitor. In order to solve this problem, a capacitorcontaining a sintered body of a powdery material having a dielectricconstant higher than that of tantalum has been studied. Niobium andtitanium are mentioned as examples of the powdery material having a highdielectric constant.

However, a sintered body of the above-described material has a highspecific leakage current index. Since niobium and titanium have highdielectric constants, a capacitor having high capacitance can beproduced from these materials, but lower specific leakage current indexis required in order to produce a capacitor of high reliability.Specific leakage current index, i.e., leakage current per unitcapacitance, can be used to evaluate whether high capacitance can beobtained while maintaining leakage current at a practically permissiblelow level.

The specific leakage current index is determined as follows. A sinteredbody having a dielectric layer formed thereon by electrolytic oxidationis prepared, and 70% of formation voltage is continuously applied to thesintered body for three minutes. The leakage current during theapplication of voltage is divided by the product of formation voltageduring electrolytic oxidation and capacitance of the sintered body.Thus, the specific leakage current index is expressed by the followingformula:Specific leakage current index=[LC/(C×V)]Wherein LC: Leakage current, C: Capacitance, and V: Formation Voltage.

In the case of a sintered body of a tantalum powder, a specific leakagecurrent index is not more than 1,500 [pA/(μF·V)], as calculated fromcapacitance and leakage current described in a catalogue entitled“CAPACITOR GRADE TANTALUM” by Showa Cabot Supermetals K.K. In order toguarantee this value, it is generally accepted that the actual measuredvalue of specific leakage current index must be at most ⅓ to ¼ of thevalue calculated from the catalogue, and a preferred leakage currentindex is not more than 400 [pA/(μF·V)]. However, a conventional sinteredbody of niobium or titanium powder has a specific leakage current indexmuch higher than the preferred leakage current index, and thus acapacitor containing the sintered body of niobium or titanium has poorreliability and is impractical for use.

SUMMARY OF THE INVENTION

In view of the foregoing, an object of the present invention is toprovide a niobium powder suitable for the production of a capacitorhaving a low specific leakage current index.

Another object of the present invention is to provide a sintered body ofniobium powder, used for a capacitor having a low specific leakagecurrent index.

A further object of the present invention is to provide a capacitor witha low specific leakage current index, which has an electrode composed ofa sintered body of niobium powder.

In a first aspect of the present invention, there is provided a niobiumpowder having a degree of nitridation represented by a nitrogen content(hereinafter referred to as “nitrogen content”) of at least about 500ppm by weight and not more than about 7,000 ppm by weight and having amean particle diameter of at least about 0.2 μm and smaller than about 3μm. Preferably the niobium powder contains as impurity at least oneelement M selected from iron, nickel, cobalt, silicon, sodium, potassiumand magnesium in an amount such that each element M is not more than 100ppm by weight, or the total amount of the elements M is not more than350 ppm by weight.

In a second aspect of the present invention, there is provided asintered body produced from a niobium powder, which exhibits a specificleakage current index of not more than about 400 [pA/(μF·V)].

In a third aspect of the present invention, there is provided a sinteredbody produced from the niobium powder concerned with the first aspect ofthe present invention.

In a fourth aspect of the present invention, there is provided acapacitor comprising (i) an electrode, wherein the electrode is thesintered body concerned with the second or third aspect of the presentinvention, (ii) a counter electrode, and (iii) a dielectric interveningbetween the two electrodes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The niobium powder of the present invention is characterized as having anitrogen content of at least about 500 ppm by weight and not more thanabout 7,000 ppm by weight and having a mean particle diameter of atleast about 0.2 μm and smaller than about 3 μm. A capacitor producedfrom the niobium powder exhibits a very low specific leakage currentindex.

The reason for which the capacitor of the niobium powder exhibits a verylow specific leakage current index is inferred below.

Generally, capacitance of a capacitor is represented by the followingformula:C=∈×(S/d)Wherein C: capacitance, ∈: dielectric constant, S: specific surface areaand d: distance between electrodes.

In the above expression, since d=k×V (k: constant and V: formationvoltage), C is represented by the following formula:C=∈×[(S/(k×V)], and thus, C×V=(∈/k)×S.

When specific leakage current index is defined by the following formulaas hereinbefore mentioned.Specific leakage current index=[LC/(C×V)](LC: leakage current), the specific leakage current index [LC/(C×V)] canbe expressed by the following formula:Specific leakage current index=LC/[(∈/k)×S].

In consideration of the above formulas, in order to decrease specificleakage current index, there may be selected any measure from amongdecreasing leakage current (LC), increasing (C×V), increasing ∈, andincreasing S.

In the present invention, the niobium powder of the present inventionhas a mean particle diameter of smaller than about 3 μm , the specificsurface area of the powder is large. Consequently, the (C×V) value,which is the denominator in the above-described formula providingspecific leakage current index, is large. However, when the meanparticle diameter of the niobium powder is smaller than about 0.2 μm, asintered body produced from the niobium powder has a problem such thatpermeation of a negative electrode material into the sintered bodybecomes difficult. As a result, capacitance of the produced capacitorcannot be increased to the desired extent, and the (C×V) value cannot bemade large, so that the sintered body is unsuitable for practical use.

Meanwhile, niobium may be bonded more strongly with oxygen than maytantalum, and thus, oxygen atoms in an electrolytic-oxidized film formedon niobium tend to diffuse toward the interior metal, i.e., niobium. Incontrast, in the sintered body according to the present invention, aniobium powder is partially bonded with nitrogen, and thus oxygen in anelectrolytic-oxidized film formed on niobium is hardly bonded withniobium, preventing diffusion of oxygen atoms toward the niobium.Consequently, the oxidized film can be stabilized and leakage current(LC) may be decreased.

In addition, since the niobium powder according to the present inventioncomprises a nitrogen content of at least about 500 ppm by weight and notmore than about 7,000 ppm by weight, leakage current, serving as thenumerator of the above-described formula, becomes especially low.Therefore, specific leakage current index of the sintered body accordingto the present invention may become particularly low.

As is described above, the sintered body of the present invention has asatisfactory specific leakage current index as low as not more thanabout 400 [pA/(μF·V)]. Furthermore, in the present invention, when anitrogen content in the niobium powder and the mean particle diameter ofthe niobium powder are optimized, the specific leakage current index maybecome not more than about 200 [pA/(μF·V)].

A niobium powder having a mean particle diameter of at least about 0.2μm and less than about 3 μm serves as a raw material for forming thesintered body. In order to decrease the specific leakage current index,the mean particle diameter is more preferably at least about 0.5 μm andless than about 2 μm. If the mean particle diameter is less than about0.2 μm, when a capacitor is fabricated from a sintered body producedfrom the niobium powder, a negative electrode material described belowbecomes difficult to soak into he sintered body because pores in thesintered body become very small. In contrast, if the mean particlediameter is about 3 μm or larger, the sintered body having a desirablespecific leakage current index is difficult to obtain. As used herein,in the case of the niobium powder, the term “mean particle diameter”refers to D₅₀ value, i.e., particle diameter value having a cumulativeweight % of 50, which is measured by a particle size distributionmeasurement apparatus (commercial name, Microtrac).

The niobium powder having the above-described mean particle diameter canbe produced by means of, for example, pulverization of a sodium-reducedcompound of potassium fluoroniobate, pulverization of a hydrogenatedniobium ingot followed by dehydrogenation, or carbon-reduction ofniobium oxide. The mean particle diameter of niobium powder can becontrolled, for example, by the degree of hydrogenation of a niobiumingot, the pulverization time, and pulverization apparatus, when theniobium powder is obtained by pulverization of hydrogenated niobiumingot followed by dehydrogenation.

The thus-obtained niobium powder may contain impurities attributed tothe raw material, the reducing agent, and the apparatus employed.Typical impurities are elements M, which include iron, nickel, cobalt,silicon, sodium, potassium, and magnesium. The above-described niobiumpowder may be washed with an alkali and at least one acid selected fromhydrofluoric acid, nitric acid, sulfuric acid and hydrochloric acid.Alternatively, the niobium powder may be washed with the above acid, analkali, and aqueous hydrogen peroxide. These reagents may be usedsequentially or in combination, so as to wash the niobium powderrepeatedly for removal of impurities. More specifically, the niobiumpowder may be sufficiently washed, for example, with sulfuric acid, andresidual sulfuric acid may be neutralized by use of an alkali, afterwhich, the niobium powder may be repeatedly washed with water. Whennitric acid is used together with hydrogen peroxide for washing theniobium powder, oxidation of the powder by nitric acid can beadvantageously prevented. The niobium powder may also be washed by meansof another method, for example, the niobium powder is stirred in theabove-described reagents for an appropriate period of time, i.e., untilthe impurity content reaches a predetermined value or less, and thepowder is separated from the reagent with stirring.

In the present invention, impurity content of the niobium powder shouldpreferably be reduced as low as possible. Generally, impurity content onthe surface of a powder increases in accordance with the surface area,and therefore, in the above-described formula for calculating “specificleakage current index,” “leakage current (LC)” serving as the numeratortends to become larger than “(C×V)” serving as the denominator. However,in the present invention, by suppression of impurity content, anincrease of “(C×V)” serving as the denominator can become larger inrelation to “leakage current (LC)” serving as the numerator, as comparedwith typical cases.

When the niobium powder containing the element M as an impurity is usedfor producing a capacitor, the element M may migrate into a dielectriclayer. Therefore, when voltage is applied to the capacitor, the elementM may cause abnormal accumulation of electric charge, and specificleakage current index of the capacitor may become larger.

The amount of each of the elements M should preferably be not more thanabout 100 ppm by weight, or the total amount of the elements M should benot more than about 350 ppm by weight. By reducing the impurity content,a baneful influence on the above-described dielectric layer can bereduced. In order to decrease the specific leakage current indexfurther, the amount of each of the elements M is preferably not morethan about 70 ppm by weight, and more preferably not more than about 30ppm by weight. Also, in order to decrease the specific leakage currentindex further, the total amount of the elements M is preferably not morethan about 300 ppm by weight, and more preferably not more than about200 ppm by weight.

The niobium powder according to the present invention has theabove-described mean particle diameter, and a nitrogen content of atleast about 500 ppm and not more than about 7,000 ppm by weight. Inorder to further reduce the specific leakage current index, the nitrogencontent is preferably at least about 1,000 ppm and not more than about3,000 ppm by weight. When the content is less than about 500 ppm byweight or in excess of about 7,000 ppm by weight, a sintered body havingthe desired specific leakage current index becomes difficult to obtain.As used herein, nitrogen content refers not to the amount of nitrogenadsorbed onto the niobium powder, but to the amount of nitrogen whichhas been chemically bound to niobium.

Liquid nitrogen, nitrogen ions, and nitrogen gas may be used as thenitrogen source for nitridation of the niobium powder, and these may beused either alone or as combination of two or more thereof. The niobiumpowder is preferably subjected to nitridation under a nitrogen gasatmosphere, since a convenient apparatus can be used with easyoperation. For example, the niobium powder is allowed to stand under anitrogen atmosphere, to thereby give a nitrided niobium powder. In thiscase, the niobium powder is allowed to stand under a nitrogen atmosphereat a temperature of not higher than about 2,000° C. for within severaltens of hours, to thereby obtain the niobium powder having the intendednitrogen content. Conducting the nitridation at higher temperature mayshorten the time for treatment.

In consideration of lower specific leakage current index, in order toobtain the niobium powder having a nitrogen content in the range ofabout 500 ppm by weight to about 7,000 ppm by weight, after the particlediameter of niobium powder is measured, nitridation temperature and timecan be controlled in relation to the particle size under conditionswhich are determined by a pre-test.

The procedure by which the niobium powder is sintered is notparticularly limited, and the conventional procedure can be employed.For example, the niobium powder is press molded into a predeterminedshape, and then maintained at a temperature of about 500° C. to about2,000° C. under a reduced pressure of about 1 Torr to about 1×10⁻⁶ Torrfor several minutes to several hours to give the sintered body.

A capacitor comprising (i) an electrode, which is the above-describedsintered body produced from a niobium powder, (ii) a counter electrode,and (iii) a dielectric intervening between the two electrodes (i) and(ii), preferably formed on the sintered body, may be produced. Apreferable example of the dielectric of the capacitor includes niobiumoxide. For example, the niobium sintered body serving as one electrode(i) is subjected to electrochemical formation in an electrolyticsolution, to thereby form a dielectric composed of niobium oxide on asurface of the sintered body. The electrochemical formation in theelectrolytic solution is typically performed by using an aqueoussolution of a protic acid, for example, an about 0.1% aqueous solutionof phosphoric acid or sulfuric acid. When a niobium oxide dielectric isformed by electrochemical formation of the niobium sintered body in theelectrolytic solution, the capacitor according to the present inventionserves as an electrolytic capacitor and the niobium sintered bodybecomes a positive electrode.

No particular limitation is imposed on the material of the otherelectrode of the capacitor according to the present invention. Forexample, there may be used at least one compound selected fromelectrolytic solutions, organic semiconductors and inorganicsemiconductors, all of which are publicly known in the aluminumelectrolytic capacitor industry. As specific examples of theelectrolytic solution, there can be mentioned a mixed solution ofdimethylformamide and ethylene glycol having dissolved therein 5% byweight of isobutyltripropylammonium borotetrafluoride, and a mixedsolution of propylene carbonate and ethylene glycol having dissolvedtherein 7% by weight of tetraethylammonium borotetrafluoride. Asspecific examples of the organic semiconductor, there can be mentionedan organic semiconductor containing benzopyrroline tetramer andchrolanyl, an organic semiconductor predominantly comprisingtetrathiotetracene, an organic semiconductor predominantly comprisingtetracyanoquinodimethane, and an organic semiconductor predominantlycomprising a conductive polymer, which is a polymer represented by thefollowing formula (1) or (2) and has been doped with a dopant. Asspecific examples of the inorganic semiconductor, there can be mentionedan inorganic semiconductor predominantly comprising lead dioxide ormanganese dioxide, and an inorganic semiconductor comprising triirontetroxide. These semiconductors may be used either alone or ascombination of two or more thereof.

In formulas (1) and (2), R¹ to R⁴ independently represent a hydrogenatom, a C₁-C₆ alkyl group or a C₁-C₆ alkoxy group; X represents anoxygen, sulfur or nitrogen atom; R⁵, which is present only when X is anitrogen atom, represents a hydrogen atom or a C₁-C₆ alkyl group; and R¹may be bonded together with R², and R³ may be bonded together with R⁴,to form a cyclic structure.

As specific examples of the polymers of formula (1) and (2), there canbe mentioned polyaniline, polyoxyphenylene, polyphenylene sulfide,polythiophene, polyfuran, polypyrrole, polymethylpyrrole, andderivatives thereof.

When, as the above-described organic semiconductor and inorganicsemiconductor, those having a conductivity of about 10⁻² S·cm⁻¹ to about10³ S·cm⁻¹ is used, the impedance of the produced capacitor furtherdecreases and the capacitance of the capacitor may be further increasedat high frequency.

When the other electrode is in solid form, carbon paste and silver pastemay be successively formed on the other electrode and encapsulated witha material such as an epoxy resin, to thereby fabricate a capacitor. Thecapacitor may have a niobium or tantalum lead, which has been sinteredtogether with the niobium sintered body or welded to the sintered bodyafter sintering. When the other electrode is in liquid form, thecapacitor comprising the above-described electrodes and the dielectricmay be placed in a can which is electrically connected to the otherelectrode for the fabrication of a capacitor. In this case, theelectrode of the niobium sintered body is led to the outside via theabove-described niobium or tantalum lead, and the lead is insulated fromthe can by a material such as insulating rubber.

As is described above, a sintered body having low specific leakagecurrent index, obtained according to the present invention, may be usedfor producing a capacitor having low leakage current and highreliability.

The present invention will now be described in more detail by thefollowing working examples.

Characteristics of a niobium powder, a sintered body and a capacitorwere determined by the following methods.

-   (1) Nitrogen Content in Niobium Powder

The amount of nitrogen bound in a nitrided niobium powder was determinedbased on thermal conductivity of the powder as measured by an oxygen-and nitrogen-measuring apparatus (supplied by LECO). The ratio (unit:ppmby weight) of the amount of bound nitrogen to the separately measuredweight of the nitrided powder was regarded the nitrogen content.

-   (2) Capacitance of Sintered Body

The capacitance of a sintered body was measured by an LCR meter(supplied by HP), which was connected between the sintered body immersedin aqueous 30% sulfuric acid and a tantalum electrode placed in asulfuric acid solution. The measurement of capacitance was conducted at120 Hz and room temperature.

-   (3) Leakage Current (LC) of Sintered Body

The leakage current (LC) of a sintered body was measured as follows. DCvoltage equivalent to 70% of the electrochemical forming voltageemployed during preparation of a dielectric was imposed for threeminutes at room temperature between the sintered body immersed in anaqueous 20% solution of phosphoric acid and an electrode placed in anaqueous solution of phosphoric acid. After voltage application, thecurrent was measured as the leakage current of the sintered body. In thepresent invention, the imposed voltage was 14 V.

-   (4) Capacitance of Capacitor

A capacitor was formed into a chip, and the capacitance of the chip wasmeasured by an LCR meter (supplied by HP) which was connected betweenterminals of the chip at room temperature and at 120 Hz.

-   (5) Leakage Current of Capacitor

The leakage current of a capacitor formed into a chip was measured asfollows. DC voltage equivalent to approximately ⅓ to ¼ of theelectrochemical forming voltage employed during preparation of adielectric was selected from rating voltages such as 2.5 V, 4 V, 6.3 V,10 V, 16 V and 25 V. The selected voltage was applied at roomtemperature between terminals of the produced chip for one minute. Aftervoltage application, the current was measured as the leakage current ofthe capacitor formed into a chip. In the present invention, the appliedvoltage was 6.3 V.

EXAMPLES 1 TO 3 AND COMPARATIVE EXAMPLE 1 to 3

Potassium fluoroniobate (20 g) which had been thoroughly dried at 80° C.in vacuum, and sodium (amount by mol of ten times that of potassiumfluoroniobate) were placed in a nickel crucible, and the mixture wasallowed to react for reduction at 1,000° C. for 20 hours under an argonatmosphere. After the completion of reaction, the reaction mixture wascooled, and the thus-reduced product was sequentially washed with water,95% sulfuric acid, and water, and then dried in vacuum. The driedproduct was pulverized by a ball mill having an alumina pot andsilica-alumina balls contained therein for 40 hours. The pulverizedproduct was immersed in a mixture of 50% nitric acid and 10% aqueoushydrogen peroxide (3:2 by weight) with stirring so as to remove animpurity introduced during pulverization. The thus-treated product wasthoroughly washed with water such that the pH of wash liquid reached 7,and dried in vacuum, to thereby obtain a niobium powder having a meanparticle diameter of 2.6 μm. The niobium powder had a mean pore diameterof 1.4 μm as measured by a mercury intrusion method.

The niobium powder was allowed to stand in a vessel filled with nitrogenfor two hours at temperatures shown in Table 1, to thereby inducenitridation. Measured nitrogen contents are shown in Table 1. The meanparticle diameter and mean pore diameter of the nitrided niobium powderwere the same as those as measured before the nitridation.

Subsequently, each of the nitrided niobium powders was shaped togetherwith a niobium wire (0.3 mmφ) into a compact having approximatedimensions of 0.3 cm×0.18 cm×0.45 cm (approximately 0.1 g). The compactwas allowed to stand in vacuum of 3×10⁻⁵ Torr at 1,300° C. for 30minutes, to thereby produce a sintered body thereof. The sintered bodywas electrochemically converted through application of 20 V in a 0.1%aqueous solution of phosphoric acid at 80° C. for 200 minutes, tothereby form a niobium oxide dielectric layer on a surface of thesintered body. Thereafter, the capacitance in 30% sulfuric acid, and theleakage current (hereinafter referred to as “LC” when appropriate) afterapplication of voltage at 14 V for three minutes at room temperature ina 20% aqueous solution of phosphoric acid were measured. The results,and a specific leakage current index are shown in Table 1.

EXAMPLES 4 TO 11 AND COMPARATIVE EXAMPLES 4 TO 6

A niobium rod (20 mmφ, 20 g) was placed in a reactor made of SUS 304.After the reactor was degassed to attain vacuum (approximately 6×10⁻⁴Torr), the temperature of the reactor was elevated to 800° C., andhydrogen was fed into the reactor. Hydrogen was further introduced at350° C. for 50 hours. After cooling, a hydrogenated niobium ingot waspulverized for 10 hours in a one-liter pot made of SUS 304 andcontaining iron balls. The pulverized product was placed into theabove-described reactor made of SUS 304, and hydrogenated under the sameconditions as mentioned above. The thus-formed hydrogenated product wasmixed with water, to prepare a 20 vol. % slurry, which was pulverizedwith zirconia balls by a wet-crusher made of SUS 304 (trade name,Attriter). The pulverizing time was varied as shown in Table 1, tothereby produce a plurality of niobium powders having different meanparticle diameters. Each of the niobium powders was sequentially washedwith 95% sulfuric acid, water, a mixture of 30% hydrofluoric acid and50% nitric acid (1:1 by weight), and water, and dried in vacuum, tothereby remove an impurity introduced during pulverization. Nitridation,shaping, and sintering were carried out in the same manner as describedin Example 1 or 2, or Comparative Example 1, to thereby produce sinteredbodies. In Examples 10 and 11, sintering was carried out at 1,200° C.Conditions for nitridation and the measured nitrogen contents are shownin Table 1. The measured capacitance, LC, and specific leakage currentindex are also shown in Table 1.

The amount of each of the elements as impurities contained in niobiumpowders of Examples 1 to 11 and Comparative Examples 1 to 6 was measuredthrough atomic absorption spectrometry. The results are shown in Table2.

TABLE 1 Examples Powder Pulveri- Mean Nitrida- and produc- zing particletion Nitrogen Capaci- Specific Comparative tion time diameter temp.content tance LC LC index Examples method [hr] [μm] [° C.] [wt. ppm][μF] [μA] [pA/ (μFV)] Com. Ex. 1 A 40 2.6 200 300 225 4 890 Example 12.6 300 1000 227 1.5 330 Example 2 2.6 400 3000 224 1 220 Example 3 2.6500 7000 220 1.5 340 Com. Ex. 2 2.6 550 14000 222 3 680 Com. Ex. 3 2.6600 20000 221 5 1130 Com. Ex. 4 B 1 3.5 200 200 174 8 2300 Com. Ex. 5 13.5 300 700 173 3 870 Com. Ex. 6 1 3.5 400 2300 175 2 570 Example 4 21.7 200 600 310 2 320 Example 5 2 1.7 300 1800 308 1 160 Example 6 2 1.7400 4500 306 2 330 Example 7 3 1.1 200 1100 450 1 110 Example 8 3 1.1300 1900 448 1 110 Example 9 3 1.1 400 5800 454 2 220 Example 10 5 0.7300 3000 590 2 170 Example 11 18 0.2 300 3800 970 3 150 Note, A:Pulverization of reduction product of potassium fluroniobate B:Pulverization of hydride of niobium ingot

TABLE 2 Content of impurity element [wt. ppm] Fe Ni Co Si Na K Mg TotalCom. Ex. 1 50 50 20 60 20 50 20 270 Example 1 50 50 20 60 20 50 20 270Example 2 50 50 20 60 20 50 20 270 Example 3 50 50 20 60 20 50 20 270Com. Ex. 2 50 50 20 60 20 50 20 270 Com. Ex. 3 50 50 20 60 20 50 20 270Com. Ex. 4 20 20 15 30 5 5 5 100 Com. Ex. 5 20 20 15 30 5 5 5 100 Com.Ex. 6 20 20 15 30 5 5 5 100 Example 4 20 20 20 30 5 5 5 105 Example 5 2020 20 30 5 5 5 105 Example 6 20 20 20 30 5 5 5 105 Example 7 20 20 20 305 5 5 105 Example 8 20 20 20 30 5 5 5 105 Example 9 20 20 20 30 5 5 5105 Example 10 30 30 20 30 5 5 5 125 Example 11 35 35 20 30 5 5 5 135

EXAMPLES 12 TO 13 AND COMPARATIVE EXAMPLES 7 TO 9

The procedures of Examples 1 to 3 and Comparative Examples 1 to 3 wererepeated in Examples 12 to 14 and Comparative Examples 7 to 9,respectively, to thereby prepare 50 sintered bodies for each of theworking examples. Each of the sintered bodies was electrochemicallyconverted at 20 V in a 0.1% aqueous solution of phosphoric acid for 200minutes, to thereby form a dielectric niobium oxide film on the surfaceof the sintered body. Subsequently, the thus-treated sintered body wasimmersed in an aqueous solution of manganese nitrate and heated at 220°C. for 30 minutes. This immersion-heating procedure was repeated, tothereby form on the dielectric niobium oxide film a manganese dioxidelayer as the other electrode. A carbon layer and a silver paste layerwere successively formed on the manganese dioxide layer. Then a leadframe was placed on the thus-produced element, and the entirety of theelement was encapsulated with an epoxy resin, to thereby produce a chipcapacitor.

Capacitance and LC value of capacitor (average value of 50 capacitors)are shown in Table 3. The LC value was measured after 6.3 V was imposedfor one minute at room temperature.

EXAMPLES 15 TO 17 AND COMPARATIVE EXAMPLE 10

The procedures of Comparative Example 5 and Examples 9 to 11 wererepeated in Comparative Example 10 and Examples 15 to 17, respectively,to thereby prepare 50 sintered bodies for each of the working examples.Each sintered body was treated in a manner similar to that described inExample 12, to thereby form a dielectric niobium oxide film on a surfaceof the sintered body. Subsequently, the thus-treated sintered body wasimmersed in a mixture of a 35% aqueous solution of lead acetate and a35% aqueous solution of ammonium persulfate (1:1 by volume) and heatedat 40° C. for one hour. This immersion-heating procedure was repeated,to thereby form on the dielectric niobium oxide film a layer comprisinga lead dioxide-lead sulfate mixture (lead dioxide content of 94 wt.%),serving as the other electrode. A carbon layer and a silver pastelayer were successively formed on the lead dioxide-lead sulfate mixturelayer. A lead frame was placed on the thus-produced element, and theentirety thereof was encapsulated with an epoxy resin, to therebyproduce a chip capacitor.

Capacitance and LC value of capacitor (average value of 50 capacitors)are shown in Table 3. The LC value was measured after 6.3 V was imposedfor one minute at room temperature.

TABLE 3 Sintered body Specific LC Capacitor chip index Capacitance[pA/(μF · V)] LC [μA] [μF] Com. Ex. 7 890 18.9 210 Example 12 330 1.0212 Example 13 220 0.9 208 Example 14 340 1.1 205 Com. Ex. 8 680 17.6218 Com. Ex. 9 1,130 34.2 206 Com. Ex. 10 870 15.0 151 Example 15 2201.6 411 Example 16 170 2.3 526 Example 17 150 3.5 809

From comparison of Examples 1 to 3 with Comparative Examples 1 to 3,comparison of Examples 4 to 6, Examples 7 to 9, Example 10 and Example11 with Comparative Example 4 to 6, in Table 1, it will be seen thatsintered bodies produced from niobium powder having a nitrogen contentof 500 to 7,000 ppm by weight and a mean particle diameter of at least0.2 μm and less than 3 μm have an excellent specific LC index. As isclear from Table 3, the LC of a chip capacitor is smaller when thespecific LC index of a sintered body is not more than 400 [pA/(μF·V)].Since the chip capacitor of the present invention has a specific LCindex not greater than a generally acceptable value, i.e.,0.01×capacitance×applied voltage, the capacitor is considerablyreliable.

Thus, a sintered body comprising the niobium powder of the presentinvention has an excellent specific LC index, and a capacitor producedfrom the sintered body has a small LC and serves as a considerablyreliable capacitor.

1. A niobium powder having a nitrogen content of at least about 500 ppmby weight and not more than about 7,000 ppm by weight, and having a meanparticle diameter of at least about 0.2 μm and less than about 3 μm,which contains as impurity at least one element M selected from thegroup consisting of iron, nickel, cobalt, silicon, sodium, potassium andmagnesium in an amount such that each element M is not more than 100 ppmby weight, and wherein the niobium powder has a CV value of from 89,600μFV/g to 194,000 μFV/g.
 2. The niobium powder according to claim 1,which has a mean particle diameter of at least about 0.5 μm and lessthan about 2 μm.
 3. The niobium powder according to claim 1, which has anitrogen content of at least about 1,000 ppm by weight and not more thanabout 3,000 ppm by weight.